Influence of Intense Pulsed UV Irradiation on the Viability and Proliferation of HeLa Cells

This paper presents viability and proliferation response of HeLa cells to intense pulsed UV (PUV) irradiation. The PUV source was driven by a pulse current capillary discharge with discharge energy of 15 J and a repetition rate of 10 pulses per second. The measured UV spectral region is from 150 to 380 nm. After cell irradiation, dead cells were identified using a propidium iodide fluorescent molecular probe, and the cell death ratio was statistically analyzed using a flow cytometer. A cell viability curve against UV pulse number was created, which shows that the threshold of the significant increase in cell mortality is 600 pulses. The effective action spectral region was less than 300 nm. PUV irradiation lethality may be due to severe intracellular damage to DNA or the membrane, which induced the cell apoptosis. The Caspase-3 activation of HeLa cells was detected as a marker of apoptosis. HeLa cells exposed to 600 UV pulses begin to show an increasing level of Caspase-3 activation compared with the sham sample. A real-time cell imaging system was employed to monitor cell proliferation over 96 h. Quantitative cell growth was examined by measuring monolayer cell confluence, and the proliferative effect was found to be in the sublethal region, which demonstrates a hormetic response of HeLa cells to PUV irradiation: Irradiation with UV pulse number between 10 and 100 promotes cell growth. The average growth rate rose to about 1.4 times that of the sham control after exposure to 50 UV pulses. When the UV pulses number exceeds 100, the toxicity of PUV tends to be severe, inhibiting cell growth and raising the cell death rate. This UV pulse number dependence suggests an accumulating effect on the cells. Application of a small number of UV pulses may activate some protein kinases and signal pathways related to cell proliferation, such as c-Jun N-terminal kinases and extracellular signal-regulated protein kinases. Further studies will examine the level of specific protein expression and actual DNA damage.